1. Field of the Invention
This invention relates generally to lighting systems and, in particular, to a system for converting light output from a point-like light source into a linear or planar light beam that can be collimated in one or more dimensions to a predetermined degree.
2. Related Art
Light generated by a laser or by accelerating charges to an extremely high speed (such as synchrotron radiation) is naturally collimated in the direction of propagation of the light. On the other hand, light generated by most light sources, such as fluorescent and incandescent lamps, is not naturally collimated in the direction of propagation of the light. However, collimation of incandescent or fluorescent light is often desirable to achieve high energy efficiency in illuminating a given area, or for other purposes such as use in a projection display.
A very small light source (e.g., a light emitting diode) can be treated as a point-like light source (i.e., a light source having very small dimensions, such that the light source can be treated as a point, that gives light in all directions). Previous systems for collimating light generated from a point-like light source have included lenses and parabolic mirrors. Such systems have been widely applied for everyday use in products such as flashlights, and have also been used for scientific applications such as mask aligners employed in making integrated circuits.
While the above-described systems are highly efficient, they are sometimes not suitable for use where a large area must be illuminated with collimated light, especially when the available space is limited. This is because conventional lenses and mirrors are bulky and require an unacceptably large amount of space. For example, the large volume occupied by the lens and mirror systems makes such systems inappropriate for certain applications where compact size is important, such as backlighted LCDs. A compact system that can convert a collimated light beam having a small cross-section to a collimated light beam having a large cross-section is also needed for making light-emitting diodes suitable for applications which need a large area illuminated. As will be seen hereinafter, in accordance with the present invention, a system for collimating light from a point-like light source requires far less space than the conventional lens and mirror systems.
Additionally, systems including lenses, mirrors and a lamp are not transparent and are therefore not suitable for situations in which the lighting system is placed between the viewer and the object to be observed ("frontlighting"). As will be seen hereinafter, in accordance with the present invention, a system for collimating light from a point-like light source can be used for frontlighting.
Furthermore, most lenses and mirrors have a circular cross-section. Thus, the generated light beam also has a circular cross-section. Where it is desired to generate a light beam having other than a circular cross-section (e.g., rectangular cross-section, or narrow strip cross-section such as used in a line scanner), the light beam cannot be efficiently transformed from a circular to non-circular shape. As will be seen hereinafter, in accordance with the present invention, a system for collimating light from a point-like light source enables generation of a non-circular light beam, such as a rectangular light beam, without significant energy loss.
A backlighting system including a light pipe with microprisms that is especially suitable for backlighting LCDs is described in commonly owned, copending U.S. patent application Ser. No. 08/049,509, entitled "Backlighting System with a Multi-Reflection Light Injection System and Using Microprisms," by Tai et al., the disclosure of which is incorporated by reference herein. The backlighting system of that application is thin and highly efficient in one-dimensional collimation of a line-like light beam generated from a miniature fluorescent lamp. A transparent frontlighting assembly which uses a light pipe and microprisms is described in commonly owned, copending U.S. patent application Ser. No. 08/255,378, entitled "A Flat and Transparent Front-Lighting System Using Microprisms," by Tai et al., the disclosure of which is also incorporated by reference herein. However, the above-described systems do not, for at least some applications, adequately enable conversion of light from a point-like light source to a linear or planar light beam having a sufficiently uniform distribution of light for some applications.